The rising popularity of electronic devices such as mobile applications has created a need for power conservation in integrated circuits. A semiconductor integrated circuit may comprise circuits demanding high operation voltages and circuits that allow for low operation voltages. For example, a core circuit may need only 1.0V operation voltage, while an I/O circuit may need 3.3V operation voltage. Typically, a circuit consumes higher power with a high operation voltage. However, early generations of integrated circuits only have the ability to provide one voltage supply to a large region, such as a design block. Some circuits have to share high operation voltage with other circuits even though they only need low voltage supply. Therefore, more power is consumed.
Several prior art documents were discussed in U.S. patent application Ser. No. 10/868,606, which application is incorporated by reference. Based on the ability to provide voltages, these prior art designs range from coarse-grain, where each design block shares a voltage, to fine-grain, where each row can have a high voltage (VddH) and/or a low voltage (VddL) as voltage supplies, to very fine-grain, where each cell in a row can have a VddH and/or a VddL. VddH and VddL are high or low voltages relative to each other, respectively, and the voltage levels are determined by specific design requirements.